1. Field of the Invention
This invention relates to a high strength spring steel useful in cars, aircraft, various industrial machines, etc.
2. Description of the Prior Art
In recent years, weight saving has been strongly demanded in cars for saving the cost of fuel. The same demand has also been growing in various structural parts or members including suspension devices. One possible approach for reducing the weight of suspension devices is to provide suspension springs with a high design stress. Strengthening the springs is effective as a weight-saving measure. Currently, Si-Mn type steel, designated SUP 7, and Si-Cr type steel, designated SUP 12, are mainly used as steel stock for suspension springs. In order to increase the design stress of these known spring steels, it is necessary to strengthen them. In general, the strength of steel materials is closely correlated with their hardness. On the other hand, there is the problem that when the hardness of the spring steels is increased, the toughness of the same is reduced, that is, reduction of the toughness is unavoidable in obtaining a hardness higher than that may be achieved in spring steels in current use. In order to ensure a sufficient reliability in spring steels, not only the hardness but also the toughness must be higher than those of currently available steels.